Substellar-Mass Companions to the K-Giants HD 240237, BD +48 738 and HD 96127

TL;DR

This paper reports the discovery of substellar companions around three giant stars, highlighting the challenges of radial velocity noise and its correlation with stellar metallicity, based on data from the Penn State-Torun Planet Search.

Contribution

It presents new detections of substellar companions to three K-giant stars and analyzes the impact of stellar noise and metallicity on radial velocity measurements.

Findings

Detected three substellar companions with masses ranging from 0.91 to over 5.3 Jupiter masses.

Identified a long-term radial velocity trend suggesting an additional distant companion.

Found that the amplitude of stellar oscillation noise is anti-correlated with stellar metallicity.

Abstract

We present the discovery of substellar-mass companions to three giant stars by the ongoing Penn State-Toru\'n Planet Search (PTPS) conducted with the 9.2 m Hobby-Eberly Telescope. The most massive of the three stars, K2-giant HD 240237, has a 5.3 MJ minimum mass companion orbiting the star at a 746-day period. The K0-giant BD +48 738 is orbited by a > 0.91 MJ planet which has a period of 393 days and shows a non-linear, long-term radial velocity trend that indicates a presence of another, more distant companion, which may have a substellar mass or be a low-mass star. The K2-giant HD 96127, has a > 4.0 MJ mass companion in a 647-day orbit around the star. The two K2-giants exhibit a significant RV noise that complicates the detection of low-amplitude, periodic variations in the data. If the noise component of the observed RV variations is due to solar-type oscillations, we show, using all the published data for the substellar companions to giants, that its amplitude is anti-correlated with stellar metallicity.